package question206;

/**
 * 206.反转链表
 * 给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。
 */
// 迭代
public class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode current = head;
        while(current != null){
            ListNode next = current.next;
            current.next = pre;
            pre = current;
            current = next;
        }
        return pre;
    }
}

// 递归
class Solution2 {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode newHead = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return newHead;
    }
}

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}

